The Inescapable Word (The Saint TV Series)

Plot
On holiday in Scotland, Simon's staying near a secret scientific base, where several of he staff's been killed. Soon after, a professor of the base is murdered, having scrawled the letters 'COP' before dying. His colleagues claim ignorance, but Templar believes one of them's the killer and enlists the help of the hotelier's daughter in solving the case.

Origin
This episode was adapted from a story with the grammatically questionable title "The Unescapable Word" from the 1957 book Thanks to the Saint.